High voltage switching tubes



Dec. 1, 1959 GP. BAVOR ET 2,915,669

HIGH VOLTAGE swncx-xmc TUBES 2 Sheets-Sheet 2 Filed April 12, 1957 FIG. 3

FIG. 4

AGENT United States Patent 2,915,669 HIGH VOLTAGE SWITCHING TUBES Gordon F. Bavor, Norwalk, 'Conn., and Jerome Pichert, Centerport, N.Y., assignors, bymesne assignments, to Raytheon Company, a corporation of Delaware I Application April 12, 1957, Serial No. 652,518 Claims. (Cl. 313297) This invention relates to high voltage switching tubes and has particular reference to switching tubes of the tetrode type which are capable of either blocking or permitting free passage of electrons in one direction upon application of high voltages across the tube.

Conventional high-voltage rectifier tubes such as have been commonly used in the past as electronic highvoltage switches generally controlled the direction of flow of electrons, permitting them to flow freely in one direction and completely blocking their flow in the reverse direction. For a number of uses, such as in radiography wherein a switching tube is used to control exposure duration of an X-ray tube, it is highly desirable to either block or permit free passage of electrons in only one direction in accordance with a switching signal from suitable timing means.

Although such action is the function of :onventional triodes, as used in radio transmission, for example, such triodes are not suitable for control of some high voltage X-ray tubes, radar circuits, and the like, partly because of their inability to handle the very high voltages involved and partly because of control circuit characteristics required. r

The prior art contains examples of attempts to combine the high-voltage capabilities of X-ray valve tubes with the grid control feature of the triode to produce a high-voltage switching tube for control of X-ray exposures. Such tubes have achieved some degree of success in conjunction with certain elaborate and complicated varieties of apparatus designed specifically for cineradiography. However, where low control-circuit power and voltage requirements and simple control circuitry are desirable, such tubes have not proved successful.

In fact, since the prior art tubes were not required to handle such high voltages as are handled by the presently described tube, triodes were generally used for controlling the particular X-ray or other apparatus with which they were used. With the advent of high voltage circuitry in some of the aforementioned X-ray, radar and other apparatus, it has become important to provide a switching tube which will possess a high degree of reliability under high voltage stresses.

We have found that a tetrode would be required to efiiciently fulfill the requirements. However, most known tetrodes capable of handling the high voltage circuitry were of large bulky proportions, and consequently were diflicult to handle and relatively expensive to manufacture. We have further found that we could provide a novel switching tube capable of handling high voltages and which is of relatively smalloverall size. This'was accomplished by providing planar electrodes which are mounted by novel supporting means on reentrant portions of the envelope. The reentrant portion at the cathode end of the envelope is trifurcated vto provide means for independently supporting the cathode and two grid electrodes, but all leads therefor penetrate an axially disposed stern press and project through the end of the Another object is to provide a switching tube of the tetrode type for controlling high voltages across the tube wherein the amplification factor is such that a low control grid voltage satisfactorily blocks up to kv. or

more.

Another object is the provision of a switching tube em-' bodying novel electrode supporting means whereby extremely accurate interelectrode spacings are obtained and maintained.

Another object is to provide a high voltage switching tube of the above character which embodies essentially planar electrode structures.

Other objects and advantages of the invention will become apparent from the following description taken in I connection with the accompanying drawings, wherein Fig. l is an axial sectional view of a switching tube embodying the invention;

Fig. 2 is a horizontal sectional view taken substantially on line 2-2 of Fig. 1 looking in the direction of the arrows;

Fig. 3 is an elevational view of the electrode structures as indicated by line 3-3 in Fig. 1; and

Fig. 4 is an enlarged axial sectional view of the grid and cathode in the tube shown in Fig. l. Referring to the drawings, the switching tube comprises a glass envelope 10 having an intermediate cylindrical portion 11 which encircles the interelectrode area I and which joins substantially coaxial anode and cathode end portions 12 and 13 of reduced diameter. The anode end portion 12 of the envelope has a coaxial reentrant portion 14 the inner circumferential end of which is sealed at 15 to a tubular metal anode support 16.

Encircling and spaced from the major portion of the support 16 is a tubular shield 17 which extends longitudinally of the envelope in coaxial relation to the support 16. One end of the shield 17 extends beyond the seal 15 into the space between the glass portions 12 and 14 and serves to shield the seal 15 from undesired electron born bardment. The other end portion 17a ofthe shield is of somewhat reduced diameter, and is sealed at its extreme end to a copper anode 18. The effective surface of the anode 18 is planar and may be provided, if desired, with a layer (not shown) of tungsten, molybdenum or other material having a higher melting point than copper. The anode is provided with an annular peripheral recess 19 for receiving the end of the shield 17, the ledge 20 thus formed thereby being adapted to overlie the end of the shield and thus produce a continuous unbroken surface without sharp edges which would produce stress points.

The anode 18 is provided with outwardly extending coaxial cylindrical portions 21 and 21a which aid in the dissipation of anode heat and also provide means by which the tube may be supported in position of use.

In assembling the anode structure with the envelope 10, the anode 18 is first assembled with the shield 17. Then the support 16, having the glass reentrant portion 14 sealed thereto, is inserted within the shield 17 and moved axially thereof to a point where the end of the support 16 rests upon an annular ridge 22 formed on the inner wall of the shield 17 in the reduced portion 17a thereof. The end of the support 16 is then brazed or otherwise vacuum-sealed to the shield,17,. and the reentrant portion Patented Dec. 1, 1959 3 14 is subsequently joined to the adjacent end of the envelope by conventional glass-working techniques.

The cathode end portion 13 of the envelope is also provided with a reentrant portion 23 which terminates in a trifurcated structure embodying three integral (:0- axial tubular supports 24, 25 and 26. The inner support 24 is, of course, of relatively small diameter and terminates in an integral stern press 27 which, with the support 24, forms part of and seals the end of the envelope.

A pair of cathode supporting rods 28 and 2 penetrate the stem press 27 and are adapted to be connected to a suitable source of energy external of the tube by means such as leads 28a and 29a. A cathode 30 is carried by the supports 28 and 29 and consists of a substantially planar coil or filament, preferably of thoriated tungsten wire, which is adapted to readily emit electrons when heated. While the cathode 30, as well as grid electrodes to be described hereinafter, is of the type known in the industry as a planar electrode, in the present embodiment at least the effective emitting portion of the coil is of meniscus shape, being bowed slightly in the direction of the anode 18 whereby the central portion of the coil is somewhat closer to the anode than are the peripheral portions, as clearly shown in Fig. 4.

The ends 31 and 32 (Fig. 4) of the filament are angled downwardly with respect to the general plane of the cathode and are affixed to the inner ends of the respective cathode supports 28 and 29. The preferred method used for securing the ends to the supports in a manner which will provide a rigid secure joint is by first covering the ends 31 and 32 with windings 33 and 34 respectively of pure tungsten wire and then welding the windings to the supports 28-29, whereupon the windings 33-34 will be integrally bonded to the ends 3132 simultaneously with the formation of the bonds between the windings and the supports.

One or more of the individual coils of the cathode 30 may be individually supported by a relatively fine wire 35 which is wound once around the coil, with its ends being affixed to an angled support wire 36 carried by the stern press 27. Such means aids in preventing damage to the cathode which might be caused by vibration or mechanical shock and furnishes additional support to aid in maintaining the meniscus shape.

The middle furcation 25 formed on reentrant portion 23 supports a control grid structure 49, and to accomplish this terminates in an annular edge to which is sealed one end of a coaxial grid-supporting metal cylinder 37 preferably formed of Kovar. To the other end of the cylinder 37 is secured one end of a tubular support 38, formed preferably of nickel, which has its other end provided with an inwardly turned flange 39. On the flange 39 is mounted the control grid structure 40 which embodies a mesh 41 sandwiched between two annular rings 42 and 43. The rings are preferably formed of molybdenum and the mesh of platinum-clad molybdenum, the platinum coating being provided to the grid wires to reduce grid emission during operation of the tube.

A suitable electrical potential can be applied to the control grid 49 through a metal connector 44, shown in Fig. 2, which is connected at one end to the Kovar cylinder 37 and at its other end to a rod 45 which penetrates the stern press 27.

A screen grid structure 46 is located between the control grid 40 and anode 18 in predetermined spaced relation thereto and also consists of a platinum-clad molybdenum wire mesh 67 which is sandwiched between two molybdenum rings 48 and 4-9. This three-part unit is rigidly secured, as by brazing, to the under side of an annular inwardly directed flange 54} which is formed on one end of a tubular nickel support 51. The support 51 is securely attached approximately midway of its length to a tubular Kovar cylinder (Figs. 1 and 4) which in turn is sealed throughout its opposite end at 53 to the outer tubular furcation 26 which forms the third part of the trifurcated reentrant portion 23 of the envelope. The portion 54 of the support 51 which extends below the joint with cylinder 52 is flared outwardly so as to be spaced nearer to the envelope than to the support 51, and then extends downwardly sufiiciently to overlie the metalto-glass seal 53.

A suitable electrical potential is supplied to the screen grid 46 through a metal connector 55 which is connected at one end to the Kovar cylinder 52. Connector 55 extends through an opening 56 in cylinder 37 and is attached at its other end to a rod 57 which penetrates the stem press 27 and is connected to a source of potential by an external lead 58.

Suitable gettering means 59 (Fig. l) is carried by the portion 54 of support 51.

On the side of flange 50 opposite the screen grid unit 46 is a reinforcing ring 60 provided to prevent deformation of the flange due to heat when the unit 46 is being brazed in place.

The grids 40 and 46 are both substantially planar-type electrodes the effective portions of which are provided with a meniscus shape, convex on the side nearest the anode in a similar manner as the cathode 30. The term planar-type is intended to define these electrodes as being of a generally flat configuration as opposed to a cylindrical configuration, and which may embody individual portions deformed slightly from a truly flat plane.

The cathode supporting rod 29 is located substantially on the common axis of the electrode structures and carries a disc-like metal shield 61 which extends transversely of the tube between the cathode 30 and the rod 57, with clearance holes being provided in the shield 61 through which extend the support wires 36 and cathode support rod 28. By its physical introduction into this area, the shield 61 functions to electrically isolate the rod 57 and adjacent portion of connector 55 from the emitting portion of the cathode, and to additionally shield the stem press from heat radiated by the cathode.

In the presently described tube, a substantial degree of success is due to the fact that the electrodes are constantly maintained at precise spaced relations. It is important, therefor, that the assembly of the electrodes be carefully controlled so that the electrodes will be initially set at the proper inter-electrode distances.

This is accomplished in a novel manner by providing the cylinders 37 and 52 with circumferential recesses 62 and 63 respectively adjacent their inner ends, which recesses provide annular ledges upon which rest the adjacent ends of the respective tubular supports 38 and 51. The tubular supports are also formed with similar annular recesses on their inner surfaces, which recesses interfit snugly with recesses 6263.

The sleeves or cylinders 37-52 are first assembled with their respective glass furcations 25-26 and the cathode 30 is mounted in the stem press 27. Then the recess 62 is formed in cylinder 37, as by a lathe operation, to an axial depth such as will space the control grid 41 at the proper distance from the cathode when the support 38 is telescoped onto the cylinder 37 to the extent that the end of the support rests upon the annular ledge or shoulder 64. The cylinder and sleeve are welded together when in these relative positions. In order to cause the end of the cylinder 37 to simultaneously engage the shoulder 66 formed by the recess in the support 38, it is of course necessary to face off the end of the cylinder when forming the recess 62.

After the control grid unit has been assembled and mounted as described, the screen grid unit is similarly assembled by forming the recess 63 to the controlled axial depth which will space the screen grid 47 at the required distance from the control grid 41 when the support 51 is telescoped onto cylinder 52 until the recesses therein interfit.

Although various high voltages of kv. or more may be applied to the tube described herein, in one satisfactorily functioning structure cutoff occurs when about 300 volts are applied to the control grid of a tube having 125 kv. applied across the anode and cathode, and about 1000 volts are applied to the screen grid 47. Under this operating condition the amplification factor is approximately 420. in this particular example, the mesh from which grids 41 and 47 are formed is .005 inch platinum-clad molybdenum wire, strands per inch. Each of the grids and the cathode is bowed to a radius of approximately 1.3 inches. The spacing between nearest surfaces of the cathode 3i) and control grid 41 is approximately .044 inch, between control grid 41 and screen grid 47 is approximately .048 inch, and between screen grid 47 and anode 18 is approximately /8 inch.

Such a tube as described herein will withstand the application of high voltages when immersed in a high dielectric oil for purposes of insulation and heat dissipation. While known prior art types of tubes fail when high voltages are suddenly applied, such as during high speed radiography, the presently described tube will perform elficiently under the same conditions. The grids and cathode of the presently described tube, due to their predetermined curvatures in a common direction, will be permitted minimal expansion in the same direction when heated, without danger of shorting, the grid leads are effectively shielded from the filament, the glass-to-metal seals are effectively shielded from electrodes of different potentials, and the electrode-supporting structures maintain the electrodes constantly and rigidly in place.

It is apparent that all of the objects of this invention have been accomplished as described, but that many modifications may be made by those skilled in the art without departing from the spirit of this invention.

We claim:

1. A tetrode for switching high voltages by means of low control grid voltage, comprising a dielectric evacuated envelope having in one end thereof a reentrant neck supporting an anode with a substantially planar effective surface and having a reentrant portion at its other end, the reentrant portion terminating in three integral radially spaced cylindrical electrode-supporting furcations extending coaxially inwardly of the envelope, the inner furcation terminating in a stem press closing the end of the envelope, a cathode mounted on the stern press in coaxial spaced relation to the anode, a pair of sleeves supported respectively at one end on the middle and outer furcations, a pair of tubular supports telescoped onto the respective sleeves and carrying substantially planar control and screen grids at their inner ends, one of said sleeves having a circumferential recess formed therein and the respective support thereon having a portion interfitting with said recess, the grid carried by the interfitting support being spaced from the other electrodes at a precise distance controlled by the axial depth of the recess, and leads extending through the stem press and connected respectively to the grids and cathode.

.2. A switching tube of the tetrode type controlling voltages as high as approximately kv. and capable of stopping flow of electrons between cathode and anode upon application to the control grid of voltage of approximately 300 volts with screen grid voltage of about 1000 volts, and at the cutoff point having an amplification factor at said voltages of approximately 420, comprising an evacuated envelope having reentrant portions at each end which respectively support substantially planar coaxial anode and cathode electrodes thereon, control and screen grid electrode structures positioned coaxially between the anode and cathode and comprising substantially planar mesh elements of approximately 15 strands per inch platinum-clad molybdenum wire mesh, the effective portions of the grid and cathode structures being bowed in the direction of the anode to a radius of approximately 1.3 inches, the spacing between nearest surfaces of the cathode and control grid being approximately .044 inch, between nearest surfaces of the control grid and screen grid being approximately .049 inch, and between nearest surfaces of the screen grid and anode being approximately inch, and leads penetrating the cathode end of the envelope and connected to the respective grid and cathode electrodes for supplying said voltages thereto.

3. A tetrode for switching high voltages by means of low control grid voltage, comprising a dielectric envelope having means at one end supporting an anode and having a reentrant portion at its other end supporting cathode, control grid and screen grid electrodes in coaxial location with respect to the anode, said reentrant portion terminating in three integral radially spaced cylindrical furcations extending coaxially inwardly of the envelope, the inner furcation terminating in a stem press, leads for the cathode and grids extending into the envelope through the stem press, the cathode being mounted upon two of said leads, a sleeve fixedly secured at one end to the end of the middle furcation and having the control grid mounted on its other end, and a tubular support fixedly secured to the end of the outer furcation and having the screen grid mounted on the end thereof ad jacent the control grid.

4. A tetrode substantially as set forth in claim 3 and having a metal connector connecting the control gridsupporting sleeve to one of the leads, and a second metal connector extending freely through an opening in the sleeve and connecting the control grid support to another lead.

5. A tetrode substantially as set forth in claim 3 and having a metal shield carried by one of the cathode leads and extending transversely of the tube between the cathode and the leads for the grids.

References Cited in the file of this patent UNITED STATES PATENTS 1,909,594 Round May 16, 1933 1,956,396 Moran Apr. 24, 1934 1,992,975 Ulrey Mar. 5, 1935 2,446,379 McArthur Aug. 3, 1948 

